User blog comment:Mega Z-Move Ultra Diancie/The Encounter rates Calculated/@comment-31267872-20180403000026

In words, that last probability is a one in 17 trillion, 83 billion, 351 million, 40 thousand chance.

17,083,351,040,000 (With Commas!)

Theoretically, if you get 17,083,351,040,000 wild encounters, you should be able to encounter a Shiny HA Roaming Legendary/Mythical with 6x31 IVs and the perfect nature without any Abilities, boosts, or gamepasses.

I'm sure that no one is even close.